This invention relates to a control system for controlling the condenser pressure in a refrigeration system. While the invention may be employed in a variety of refrigeration systems, it is particularly useful in all-weather air conditioning equipment required to operate in the presence of broad range of outside ambient temperatures, and will be described in that environment.
The condenser coil of an air conditioning system is usually located out-of-doors or in heat exchange relation with outdoor air and is therefore subjected to widely varying ambient temperatures. If the system operates during cold weather, the outdoor temperatures may drop sufficiently low to materially reduce the condensing temperature of the refrigerant in the condenser coil. This produces a corresponding reduction in head pressure on the high pressure side of the refrigeration system, resulting in a decreased pressure differential across the thermal expansion valve or other refrigerant metering device in the system. Because of the reduced pressure difference across the metering device, less refrigerant flow from the condenser to the evaporator. The capacity of the refrigeration system is accordingly reduced and the cooling load placed on the evaporator may not be satisfied.
In some instances, the reduction in head pressure at low ambient temperatures may result in the evaporator coil being cooled to a temperature below freezing, allowing condensed moisture to freeze on the evaporator coil. As the layer of ice builds up on the evaporator coil, the coil becomes insulated from the refrigeration load and a further reduction in system capacity occurs.
Systems have been developed for preventing a pressure drop on the high pressure side of the refrigeration system, thereby to maintain the minimum pressure differential across the metering device required for efficient operation, by reducing the speed of at least one fan motor for the condenser as the ambient temperature falls. The volume of air blown across the condenser coil therefore decreases and this limits the amount of heat that can be extracted from the refrigerant as it passes through the condenser coil, insuring that the refrigerant temperature, and consequently its pressure, does not fall below the required minimum. With the pressure on the high side of the system at or above the minimum, the pressure difference across the expansion or metering device will be at or above the level necessary for efficient operation.
The present invention also maintains a minimum head pressure by keying the condenser fan speed to condensing temperature. These functions are achieved, however, by means of a control system considerably simpler, more reliable, and less expensive than those developed heretofore. Moreover, the present control system exhibits a significant improvement in performance over the prior systems.